The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, three-piece or multi-layered golf balls. One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded thereon. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance especially in terms of the distance such balls travel and the feel transmitted to the golfer through the club on striking the ball, the basic two piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer a so called “three-piece ball” results and similarly if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on.
However, the starting point and key to the performance of any golf ball is the nature of the rubber compositions used in the construction of the golf ball, and in particular the rubber hardness, compression, resilience and durability. Most modern golf balls now utilize core compositions made from synthetic rubbers based on polybutadiene, especially cis-1,4-polybutadiene. In order to tailor the properties of the core, the polybutadiene is often further formulated with crosslinking agents, such as sulfur or peroxides, or by irradiation, as well as co-crosslinking agents such as zinc diacrylate. In addition, the weight and hardness of the core may be further adjusted by the incorporation of various filler materials in the rubber formulation. Thus, there is a great deal of literature concerning such formulation chemistry and the variation of the rubber composition and degree of cross linking such that cores may be produced with the required compression, resilience, hardness and durability.
For example, U.S. Pat. No. 4,726,590 discloses a composition for one-piece golf ball cores having improved resilience. The core composition includes the following components: an elastomer cross-linkable with a free radical initiator catalyst, a metal salt of an alpha-acrylate or methacrylate, a free radical initiator catalyst, and a polyfunctional isocyanate.
U.S. Pat. No. 4,838,556 discloses a solid golf ball having a solid core comprised of an elastomer or admixture of elastomers, at least one metal salt of an unsaturated carboxylic acid, a free radical initiator, and a dispersing agent. U.S. Pat. No. 4,852,884 discloses a golf ball core formulation, which incorporates a metal carbamate accelerator. U.S. Pat. No. 4,844,471 discloses a golf ball core composition including dialkyl tin fatty acid. Finally, U.S. Pat. No. 4,546,980 discloses a golf ball core, which contains two or more free radical initiators, at least two of which exhibit a different reactivity during the curing process.
Typically, the most common method for preparing and crosslinking the polybutadiene in such cores employs a compression molding process. This choice of molding method is dictated by the relatively high viscosity of the base polybutadiene at the crosslinking temperature, which must roughly correspond to the decomposition temperature of the chemical crosslinking agent. In view of the relatively high viscosity of cis 1,4-polybutadiene at the typical decomposition temperature of most commercially available peroxides, a compression molding process is the most commercially viable process for such core preparation. This process is preferred for such high viscosity compositions, as it does not require the material to flow into the mold; rather a slug comprising the mixture of polybutadiene, crosslinking agents, fillers and any other additives are placed in the open mold halves. The mold is then closed and the materials subjected to a molding cycle in which heat and pressure are applied while the mixture is confined within a mold. The compression and heat decompose the peroxide and/or other crosslinking agents, which in turn initiate cross-linking of the rubber. The temperature, pressure and duration of the molding cycle, in addition to the nature and relative amounts of rubber crosslinking agents and other fillers and additives, can all be independently varied to control the resulting core properties.
After core formation, the golf ball cover and any intermediate layers are typically positioned over the core using one of three methods: casting, injection molding, or compression molding. Injection molding generally involves using a mold having one or more sets of two hemispherical mold sections that mate to form a spherical cavity during the molding process. The pairs of mold sections are configured to define a spherical cavity in their interior when mated. When used to mold an outer cover layer for a golf ball, the mold sections can be configured so that the inner surfaces that mate to form the spherical cavity include protrusions configured to form dimples on the outer surface of the molded cover layer. When used to mold a layer onto an existing structure, such as a ball core, the mold includes a number of support pins disposed throughout the mold sections. The support pins are configured to be retractable, moving into and out of the cavity perpendicular to the spherical cavity surface. The support pins maintain the position of the core while the molten material flows through the gates into the cavity between the core and the mold sections. The mold itself may be a cold mold or a heated mold
In contrast, compression molding of a ball cover or intermediate layer typically requires the initial step of making half shells by injection molding the layer material into an injection mold. The half shells then are positioned in a compression mold around a ball core, whereupon heat and pressure are used to mold the half shells into a complete layer over the core, with or without a chemical reaction such as crosslinking. Compression molding also can be used as a curing step after injection molding. In such a process, an outer layer of thermally curable material is injection molded around a core in a cold mold. After the material solidifies, the ball is removed and placed into a mold, in which heat and pressure are applied to the ball to induce curing in the outer layer.
Of the various cover molding processes, injection molding is most preferred, due to the efficiencies gained by its use including a more rapid cycle time, cheaper operating costs and an improved ability to produce thinner layers around the core and closely control any thickness variation. This latter advantage is becoming more important with the developments of multi-layered balls with two or more intermediate layers between the core and cover thus requiring thinner layer formation.
Like golf ball cores, golf ball covers and/or intermediate layers are sometimes made from rubber. Earlier balls almost exclusively had covers made from naturally occurring balata rubber. Many players still favor this cover material as its softness allows them to achieve spin rates sufficient to allow more precise control of ball direction and distance, particularly on shorter approach shots. However one deficiency of balata is the ease with which it is cut or sheared leading to low durability of the ball. Also, as with synthetic 1,4-polybutadiene rubber, balata rubber has relatively high viscosity at normal injection molding temperatures and thus is not easily adaptable to traditional thin layer-forming injection molding techniques. Thus the current evolution in golf balls technology favors the use of thermoplastic materials such as ionomers or thermoplastic polyurethane in golf ball covers and intermediate layers, which materials are much more amenable to modern thin layer injection molding techniques.
However, in addition to the polybutadiene-based synthetic rubbers, another synthetic rubber available for use in golf balls, are the so-called “polyalkenamers”. These synthetic rubbers are unique in that in addition to a liner polymeric component they also contain a significant fraction of cyclic oligomer molecules, which in turn lowers their viscosity. Compounds of this class can be produced in accordance with the teachings of U.S. Pat. Nos. 3,804,803, 3,974,092 and 4,950,826, the entire contents of all of which are herein incorporated by reference.
To date, this material has been utilized primarily in blends with other polymers. For instance, U.S. Pat. No. 4,183,876 describes compositions comprising 15-95 parts by weight crystalline polyolefin resin and correspondingly 85-5 parts by weight cross-linked polyalkenamer rubber per 100 total parts by weight of resin and rubber. The resulting moldable thermoplastic compositions were said to exhibit improved strength and greater toughness and impact resistance than similar compositions containing substantially uncross-linked rubber. U.S. Pat. No. 4,840,993 describes a polyamide molding compound consisting of a mixture of 60 to 98% by weight of (A) a polyamide and (B) 2 to 40% by weight of a polyalkenamer, wherein the mixture is treated at elevated temperatures with 0.05 to 5% by weight of the sum of components (A) and (B) of an organic radical former. No mention was made of the use of such compositions in balls including golf balls.
However, there a number of applications of polyalkenamer blends in game balls of various kinds. For example, U.S. Pat. No. 5,460,367 describes a pressureless tennis ball comprising a blend of trans-polyoctenamer rubber and natural rubber or other synthetic rubbers, e.g. cis-1,4-polybutadiene, trans-polybutadiene, polyisoprene, styrene-butadiene rubber, ethylene-propylene rubber or an ethylene-propylene-diene rubber (EPDM).
Also, U.S. Pat. No. 4,792,141 describes a golf ball comprising a core and a cover wherein the cover is formed from a composition comprising about 97 to about 60 parts balata and about 3 to about 40 parts by weight polyoctenylene rubber based on 100 parts by weight polymer in the composition. This patent also discloses that using more than about 40 parts by weight of polyoctenylene based on 100 parts by weight polymer in the composition has been found to produce deleterious effects.
However, it would be highly advantageous to have an injection moldable rubber composition with the soft feel of a rubber such as balata, but of sufficiently low viscosity to allow the material to be injection molded. It would also be highly advantageous if the properties of such a rubber composition could be tailored by similar formulation chemistry to that which has evolved through the use of crosslinked filled polybutadiene compositions used in core construction. In particular, it would be advantageous to have a composition that exhibits both superior toughness (e.g., durability) and high hardness. It would also be highly advantageous if such a composition could be used in a process to make a golf ball which process would include primarily injection molding to fabricate a core, outer cover and/or intermediate layer. It would also be highly advantageous if such a fabrication process would also allow formation of thin outer cover and/or intermediate layers, while also providing facile control not only of layer thickness and thickness uniformity, but while also allowing ease in variation of the resulting ball properties.
The present disclosure provides a golf ball comprising an injection moldable polyalkenamer rubber composition for use in a core, intermediate layers and/or outer cover layer of a golf ball. The present disclosure also provides a golf ball comprising a polyalkenamer/polyamide rubber composition for use in a core, intermediate layers and/or outer cover layer of a golf ball. The properties of the composition may be easily tailored for the particular golf ball component to be made by variation in the curative package employed and/or the molding conditions.
The present disclosure also provides processes for preparing a golf ball by injection molding one or more of the core, intermediate layers and/or outer cover layer of a golf ball. In one embodiment, the process utilizes a composition comprising a polyalkenamer rubber having a sufficiently low viscosity at and below normal peroxide decomposition temperatures to allow the material to be injection molded to form a core, intermediate and/or cover layer. In another embodiment, the process utilizes a composition comprising at least one polyalkenamer rubber having a sufficiently low viscosity to allow the material to be injection molded to form a core, intermediate and/or cover layer, and at least one polyamide.